10.1117/2.1201304.004773
High-speed wireless networking
using visible light
Harald Haas
White LEDs hold promise as a key enabler for future wireless networks
based on optical attocells, offering significant improvements to indoor
data coverage.
The advent of the first cellphones in the 1980s marked the beginning of commercial mobile communications. Now, only 30
years later, wireless connectivity has become a fundamental part
of our everyday lives and is increasingly being regarded as an
essential commodity like electricity, gas, and water. The technology’s huge success means we are now facing an imminent shortage of radiofrequency (RF) spectrum. The amount of data sent
through wireless networks is expected to increase 10-fold during the next four years.1 At the same time, there isn’t enough
new RF spectrum available to allocate. In addition, the spectral
efficiency (the number of bits successfully transmitted per Hertz
bandwidth) of wireless networks has become saturated, despite
tremendous technological advancements in the last 10 years. The
US Federal Communications Commission has therefore warned
of a potential spectrum crisis.
Light fidelity (Li-Fi),2 the high-speed communication and networking variant of visible light communication (VLC),3 aims to
unlock a vast amount of unused electromagnetic spectrum in
the visible light region (see Figure 1). Li-Fi works as a signal
transmitter with the off-the-shelf white LEDs typically used for
solid-state lighting and as a signal receiver with a p-i-n photodiode or avalanche photodiode. This means that Li-Fi systems
can illuminate a room and at the same time provide wireless
data connectivity. Unlike laser diodes, the LEDs my colleagues
and I studied produce incoherent light, which means the signal phase cannot be used for data communications. Therefore,
the only way to encode data is to use intensity modulation and
direct detection. This poses severe restrictions on the data rates
we can achieve.
However, it has been shown in a hardware proof-of-concept
demonstration that the high peak-to-average ratio of the signal in orthogonal frequency division multiplexing (OFDM), typically a disadvantage in RF communications, can be turned into
Figure 1. The electromagnetic spectrum and the vast potential of unused, unregulated, safe green spectrum in the visible light part. The visible light spectrum is 10,000 times larger than the entire radiofrequency
spectrum.
an advantage for Li-Fi.4 The applicability of multi-carrier modulation techniques such as OFDM has now been proven widely
on the link level, which is a single point-to-point transmission
from one transmitter to one receiver, and data rates beyond
500Mbps have been reported from a single white LED using offline processing.5 Indeed, real-time video streaming from a white
LED has been demonstrated at data rates up to 130Mbps.2 Communication networks typically have multiple links that can be
point-to-multipoint as well as multipoint-to-point.
While link-level demonstrations are important steps to prove
that Li-Fi is a viable technique to help mitigate RF communications spectrum bottlenecks, it is important to show that
fully-fledged optical wireless networks can be developed using
existing lighting infrastructures. Therefore, we investigated the
networking aspects of Li-Fi.6 This includes multi-user access
techniques and interference coordination.
If we equip a room with multiple light fixtures that each function as a very small radio base station, the result is a network
of very small cells that we call ‘optical attocells.’ They are analogous to femtocells in RF communications, which cover a small
area and are, therefore, classed as ‘small cells.’ Small cells have
been the major contributors to the improvements of three orders of magnitude reported in spectral efficiency gains in wireless communications during the past 50 years. It is, therefore, a
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10.1117/2.1201304.004773 Page 2/3
logical step to consider even smaller cells such as optical attocells, which have the added advantage of not interfering with
RF-based wireless networks.
A single room can be served by multiple optical attocells, with
each covering an area of 1–10m2 and distances of about 3m. The
main limiting factor is interference. Therefore, colleagues and I
studied the area spectral efficiency (ASE) in bits/s/Hz/m2 of an
indoor office environment, assuming a network of RF femtocells
within the office block. We compared the attained ASE with that
of a network of optical attocells where the light fixtures served
two purposes: optical access points and illumination units ensuring a minimum of 400lx (as required for reading).7
The indoor cells were surrounded by a macrocellular network
that assumed use of the long-term evolution (LTE) standard also
common in mobile phones. The femtocells used the same RF frequency spectrum as the macrocell base station, and so the femtocell network suffered from macrocell interference (see Figure 2).
We plotted the ratios of the ASEs attained for the attocell and
femtocell networks for a varying number of femtocells per floor
(see Figure 3). The ASE gain of the attocell network is higher
for smaller rooms. This is because the femtocell network suffers
from additional wall losses, whereas the attocell network benefits from complete interference protection by walls, which do
not allow light to propagate through them. The gains diminish
as the number of femtocells per floor increases, but the gain is
still about 12 (i.e., the area spectral efficiency of the optical attocell network is 12 times that of the RF femtocell network) for
Figure 2. Simulation of a three-floor office building surrounded by
seven macrocell base stations using the long-term evolution (LTE)
standard. Floor loss (FL) is 17dB, internal wall loss is 12dB and external wall loss is 20dB. Users are randomly distributed in the building.
Femto AP: Femtocell access point. BS: Base station.
Figure 3. The area spectral efficiency (ASE) of the attocell network is
divided by the ASE of the femtocell network. Results from different
room sizes are shown.
the largest room and 20 femtocells per floor. The maximum gain
in ASE is about 920 for 4 femtocells per floor, which suggests
that using optical attocells could achieve another three orders of
magnitude improvement in spectral efficiency. This could be realized within the next 2-5 years, with the additional benefit that
optical attocells use a different part of the electromagnetic spectrum, thus offloading traffic from the RF systems. This multiplicative effect significantly contributes to solving the looming
spectrum crisis.
An optical attocell network with an ASE of 1.2 bit/s/Hz/m2
and a 10MHz bandwidth in a 12.5m2 room would allow users to
share on average a total of 150Mbps from our attocell network.
If we assume 20 femtocells per floor, the attainable capacity for
the same room (2.5M x 5m) is about 1.5Mbps. While we used the
same bandwidth for both systems, the optical attocell network
requires more power, determined by the minimum illumination
requirement. Piggy-backing the communication functionality on
energy-efficient room lighting makes this a very energy-efficient
method of wireless communication.
In summary, we have shown that significant improvements of
indoor data coverage can be expected from an optical attocell
network. The optical attocell network can perfectly complement
RF systems as there is no interference between the systems. As
a next step, we intend to investigate hybrid RF femtocell and
optical attocell networks. Our goal is to enable seamless interoperation between optical attocells and RF femtocells/macrocells
to ensure maximum spectrum relief for the RF systems.
We are grateful for support from the UK’s Engineering and Physical
Sciences Research Council (EP/K008757/1).
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Author Information
Harald Haas
The University of Edinburgh
Edinburgh, United Kingdom
Harald Haas was an invited speaker at TED Global 2011, where
he first introduced Li-Fi. In 2012 he was awarded the prestigious Established Career Fellowship from the UK Engineering
and Physical Sciences Research Council. He is cofounder and
chief technology officer of pureVLC Ltd.
References
1. Homepage of the Cisco Visual Networking Index showing a forecast for global
mobile data traffic 2012–2017. http://bit.ly/132wSvd
2. TED Talk online by Harald Haas on wireless data from every light bulb.
http://bit.ly/tedvlc
3. T. Komine and M. Nakagawa, Fundamental analysis for visible-light communication system using LED lights, IEEE Trans. Consumer Electron. 50 (1), pp. 100–107,
February 2004.
4. M. Afgani, H. Haas, H. Elgala, and D. Knipp, Visible light communication using
OFDM, Proc. 2nd Int’l Conf. Testbeds Res. Infrastruct. Dev. Networks Communities, pp. 129–134, Barcelona, Spain, March 2006.
5. J. Vucic, C. Kottke, S. Nerreter, K. D. Langer, and J. W. Walewski, 513 Mbit/s visible light communications link based on DMT modulation of a white LED, J. Lightwave
Technol. 28 (24), pp. 3512–3518, December 2010.
6. L. Hanzo, H. Haas, S. Imre, D. O’Brien, M. Rupp, and L. Gyongyosi, Wireless
myths, realities and futures: from 3G/4G to optical and quantum wireless, Proc. IEEE
100, pp. 1853–1888, May 2012.
7. I. Stefan, H. Burchardt, and H. Haas, Area spectral efficiency performance comparison
between VLC and RF femtocell networks, 2013. Paper accepted at the IEEE Int’l Conf.
Commun. in Budapest, Hungary, 9–13 June 2013.
c 2013 SPIE